1. Field of the Invention
The present invention relates to a focus detection apparatus and, more particularly, to an improvement in a focus detection apparatus suitable for a photographic camera, a video camera, or the like to detect the imaging state of an imaging optical system in accordance with the relative positional relationship between two object images formed by a distance measuring optical system using a beam passing through a phototaking lens (imaging optical system).
2. Related Background Art
A focus detection apparatus of a so-called image correlation scheme as a conventional method of performing focus detection in a focus detection apparatus is disclosed in U.S. Pat. Nos. 4,559,446, 4,833,313, and many other patents. This scheme has an assumption that two object images (secondary images) to be compared with each other are identical images laterally deviated from each other. For this reason, the aperture ratio of the imaging optical system or the location of the distance measuring field is limited so as not to vignette a distance measuring beam.
High-pass filter processing is known to effectively improve precision of an image signal degraded by vignetting of the distance measuring beam. However, this processing mainly aims at eliminating small adverse influences of vignetting caused by manufacturing errors. The effect of high-pass filter processing cannot perfectly eliminate the limitations of the open f-number or allow free layout of the distance measuring field.
In a camera system obtained by applying this focus detection apparatus to, e.g., a single-lens reflex camera, open f-numbers of various phototaking lenses are assumed to be generally set to fall within the range from F1.0 to F8. For this reason, when this camera system is to be arranged as an auto-focus camera system, the open f-numbers of phototaking lenses used in the auto-focus operation are limited to ones smaller than F5.6. Alternatively, the distance measuring field is located at a position near the optical axis, e.g., a position within the range of 5 mm from the optical axis so as to reduce vignetting of the distance measuring beam. It is, therefore, very difficult to realize a camera system having an auto-focus mode which can use any kind of phototaking lens and can operate (distance measurement) in any area of the finder field.
The present invention is related to an improvement of the invention of U.S. Ser. No. 723,962 filed by the present applicant.